Although it's never been tested, the JR and order of annexation stipulate that Texas can form new states with only the approval of its state legislature. Congress' approval is not required for the division. Under the agreement, those new states are legally "entitled" to admission to the Union.
To formally admit those new states to the Union, Congress would have to follow Article IV. It seems like a distinction without a difference, but the difference, theoretically anyway, is that other states require Congress' approval before they divide and new states are not automatically entitled to admission.
In the end, it matters not. Texas is not going to divide.
If that treaty can trump the Constitution then all treaties can trump the Constitution. And if that is true then Texas has rights under the Constitution denied to other states. I don't agree with that.